|Quoting DfwRevolution (Reply 17):|
Speedometers must be accurate within a certain margin by law whereas there is no such requirement for GPS navigation systems. Look into how both devices are measuring velocity and there is no question that the properly-functioning speedometer is the more accurate device.
I won't argue that -when manufactured- or perhaps even -when originally sold- automobile speedometers are required to have a certain accuracy, however, I can tell you that in nearly 10 years of owning my car I've never been required to have the speedometer recalibrated -- or even checked -- either as part of the registration renewal, emissions testing, or any of the other things that I am required by law to do to my car on a regular basis.
(I will say that every police crusier I've sat in the driver seat of [but never driven] I've noticed the notation on the speedometer legend "Calibrated" and my understanding is that -- at least in California, those speedometers are recalibrated on at least an annual basis. I suspect that may have to do with the use of "pacing" as a speed enforcement technique and the need to ensure that the speed you claim someone is going is accurage, but I digress)
I will also argue that if you're measuring distance using a spinning round object, the distance traveled is affected by the diameter of that spinning object -- which can be affected from (duh) the diamater of the tire, to tread wear (though I doubt that by itself would be a major difference) to inflation and even temperature (as air in tire gets warmer, it expands, increasing the diamater of the tire), etc.
On the other hand my GPS (A Garmin StreetPilot something-or-other) has no moving parts, and [generally] has at least three points to reference its position off of and is remarkibly accurate positionally, as long as it has a clear view of the sky [Downtown Cleveland, for example, can confus it] -- I've marked points of interest and it has returned me to the _exact_ spot (+/- about 10') on a consistent basis -- Cleveland has it's share of whacked out streats (intersections going 5 or 7 different ways, streets closely paralleling other streets, etc.) and with few exceptions it's managed to figure out the street I'm really on vs the one on the other side of the hedge going the same direction.
Every time I've tested my StreetPilot and Speedometer against a speed measuring device, the StreetPilot has been dead on, and my car has been around 5 miles over -- for example, sign on side of road tells me I'm going 64, Garmin tells me I'm going 64, Speedometer tells me I'm going 69. Every time I've done the test I've had the same result.
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